As far as I can see, Opera's adoption of WebKit can only result in more bug fixes submitted for the framework.
So if you remove some coders from the common codebase and shift them towards other parts of the browser that have more to do with your specific brand (JSC in Apple's example, V8 or cloud integration in Google's), up to a certain point you gain a net benefit - your brand develops faster and your partners invest more.
Of course, if each party acts like that, the common project gets stale - the tragedy of the commons.
From my completely unfair outsider view, it seems to me that Apple, for example, has more or less lost interest in WebKit and tries to stifle progress to defend their lackluster Safari against Chrome and MobileChrome.
I use both Safari and Chrome (and WebKit nightlies). I vastly prefer Safari to Chrome, but their underlying use of WebKit seems quite similar.
Competition has seemed to help WebKit in the past (especially between Apple and Google). If you recall, they both contributed a new process model to WebKit (http://betanews.com/2010/04/09/the-big-change-coming-to-safa...). It could be argued that Apple's contribution in this area was more useful to users of the WebKit framework, while Google's was restricted to Chromium. That said, Google makes many great contributions too.
Also he does compare WebKit to oldIE, though I wonder is Gecko any better.